In an antenna known as a base station antenna, a plurality of antenna elements is arranged in multiple stages in a linear pattern to achieve high-gain and omnidirectional characteristics in a horizontal plane and radiation directional characteristics of a sharp beam. Such an antenna is divided into a series feed type antenna of feeding a plurality of antenna elements connected in series and a parallel feed type antenna of feeding a plurality of antenna elements by distributing power between the antenna elements. The directional characteristics of the antenna are determined in response to the amount of exciting power (amplitude value) to be fed to each antenna element and an excitation phase.
FIGS. 41(a) and 41(b) are a front view and a bottom view respectively showing the structure of a conventional two-stage collinear antenna 200 as a series feed type antenna.
The conventional two-stage collinear antenna 200 shown in FIGS. 41(a) and 41(b) has a stack of a first-stage sleeve element 210 and a second-stage sleeve element 211 each forming a dipole antenna. The first-stage sleeve element 210 is formed of a dipole antenna including a cylindrical upper sleeve pipe 210a and a cylindrical lower sleeve pipe 210b facing each other. Likewise, the second-stage sleeve element 211 is formed of a dipole antenna including a cylindrical upper sleeve pipe 211a and a cylindrical lower sleeve pipe 211b facing each other. The upper sleeve pipes 210a and 211a and the lower sleeve pipes 210b and 211b forming the dipole antennas have an electrical length of about λ/4, where λ is the wavelength of a usable frequency. The first-stage and second-stage sleeve elements 210 and 211 are fed in series through two feeding cables including a first feeding cable 212 and a second feeding cable 213 used for feeding frequency signals of different frequencies. The first and second feeding cables 212 and 213 are each passed through the first-stage and second-stage sleeve elements 210 and 211. The electrical length of each of the first and second feeding cables 212 and 213 between respective feeding points of the sleeve elements in the first and second stages are determined to be about an integral multiple of the wavelength of a frequency signal being transmitted. In this way, the first-stage and second-stage sleeve elements 210 and 211 are fed in phase with different frequency signals. As a result, radiation patterns appropriate for communication can be obtained at two frequencies.